1. Field of the Invention
The present invention relates to an electroacoustic transducer for converting electric signals into sound through electromagnetic conversion.
2. Description of the Related Art
FIGS. 20 and 21 show a typical internal structure of a conventional electroacoustic transducer. The elctroacoustic transducer is provided with a casing 104 consisting of an upper case 100 and a lower case 102. The lower case 102 is provided with a step 106 for supporting a diaphragm 108 in such a way as to cover the lower edge of the upper case 100. The diaphragm 108 is made of a magnetic material, and provided with a magnetic piece 110 as a weight at the center thereof.
The lower case 102 is provided with a base 112, constituting a portion of a yoke and forming a laminate structure together with a bottom plate 114, and a magnetic core 116 is set up upright at the center of the base 112 in such a way as to penetrate through the bottom plate 114. Between the magnetic core 116 and the diaphragm 108, a gap 118 is provided. A coil 120 is wound around the magnetic core 116, and a length of a portion of the magnetic core 116, wound around with the coil 120, is slightly short of the uppermost end of the magnetic core 116, thereby exposing a portion of the magnetic core 116 above the coil 120. An annular magnet 122 is installed around the periphery of the coil 120 with a space provided therebetween. Further, terminals of the coil 120 are extended outside the casing 104 through through-holes 128, and connected to bar-shaped terminals 124, 126, provided on the bottom plate 114 in a projecting manner, by means of soldering on the underside of the bottom plate 114. The through-holes 128 are filled up with insulating resin for providing insulation, protection, securement of the terminals of the coil, and sealing.
A resonance chamber 130 surrounded by the upper case 100 is formed on the upper side of the diaphragm 108, and open to the air through a sound emitting hole 134 of a sound emitting cylinder 132 formed on the topwall of the upper case 100.
In the electroacoustic transducer described above, a magnetic circuit is set up by the base 112, the magnetic core 116, the diaphragm 108, and the magnet 122. A static magnetic field, caused by the magnet 122 acts on the diaphragm 108, and the magnetized diaphragm 108 is attracted towards the magnetic core 116. In other words, the magnet 122 is caused to act on the diaphragm 108 as a bias magnetic field. When electric signals such as a-c, pulse, or the like are applied between the terminals 124 and 126 against a magnetic field in one direction caused by the static magnetic field as described above, signal current flows in the coil 120, generating an alternating magnetic field in the magnetic core 116, according to the electric signals. The diaphragm 108 is caused to deflect in a direction away from the magnetic core 116 during a period when the direction of the alternating magnetic filed is opposite to that of the static magnetic field of the magnet 122 while same is attracted towards the magnetic core 116 during a period when the direction of the alternating magnetic filed is the same as that for the static magnetic field. Such mechanical movement of the diaphragm 108 up and down as described above is dependent on frequency of the electric signals, and consequently, the diaphragm 108 is vibrated, causing the air to be vibrated. Such vibration of the air is amplified in the resonance chamber 130 in the form of sympathetic vibration (resonance) acoustic waves. The acoustic waves are emitted to the outside mainly through the sound emitting hole 134.
With reference to the elctroacoustic transducer described above, problems to be solved are cited as follows:
(1) Due to a long distance between the coil 120 and the terminals 124, 126, and exposure of the connecting portion of the terminals of the coil 120 on the underside of the casing 104, the exposed terminals of the coil are susceptible to damage, creating a cause for accidental breaks of wiring. As a result, protective measures such as coating with insulating resin, or the like were required.
(2) As a connecting means for the terminals of the coil is provided outside of the casing 104, the thickness of the electroacoustic transducer, in the direction of its height, increased, preventing the electroacoustic transducer from being rendered thinner or miniaturized.
(3) In some of the electroacoustic transducer, a space is provided by cutting a notch in part of the annular magnet such that connection of the terminals of the coil with the terminals 124,126, can be made inside the casing 104. However, it is not easy to machine the magnet 122, and moreover, providing a notch severing a magnetic path in part of the magnet 122 results in lowering of magnetic attraction between the magnet 122 and the diaphragm 108. Further, a force to support the diaphragm 108 by the notched portion of the magnet 122 is partly reduced. Thus, machining the magnet 122 leads to an increase in the cost of manufacturing the electroacoustic transducer by the cost of such machining, and to partial reduction in the force to support the diaphragm 108, that is, partial change in the rigidity of the diaphragm 108, resulting in unstable vibration and consequently, degradation in the quality of sound.
(4) The height of the coil 120 is substantially equal to that of the magnet 122, and the height of the magnet has been a stumbling block for miniaturization and flattening of the electroacoustic transducer. Magnetic force generated by a coil is generally dependent on input electric current and the number of turns of a winding. The magnetic force of the magnet is very strong in comparison with that generated by the coil but this requires a relatinely large magnet.